docs/manual/mod/mod_session.xml

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<modulesynopsis metafile="mod_session.xml.meta">

<name>mod_session</name>
<description>Session support</description>
<status>Extension</status>
<sourcefile>mod_session.c</sourcefile>
<identifier>session_module</identifier>
<compatibility>Available in Apache 2.3 and later</compatibility>

<summary>
    <note type="warning"><title>Warning</title>
      <p>The session modules make use of HTTP cookies, and as such can fall
      victim to Cross Site Scripting attacks, or expose potentially private
      information to clients. Please ensure that the relevant risks have
      been taken into account before enabling the session functionality on
      your server.</p>
    </note>

    <p>This module provides support for a server wide per user session
    interface. Sessions can be used for keeping track of whether a user
    has been logged in, or for other per user information that should
    be kept available across requests.</p>

    <p>Sessions may be stored on the server, or may be stored on the
    browser. Sessions may also be optionally encrypted for added security.
    These features are divided into several modules in addition to
    <module>mod_session</module>; <module>mod_session_crypto</module>,
    <module>mod_session_cookie</module> and <module>mod_session_dbd</module>.
    Depending on the server requirements, load the appropriate modules
    into the server (either statically at compile time or dynamically
    via the <directive module="mod_so">LoadModule</directive> directive).</p>

    <p>Sessions may be manipulated from other modules that depend on the
    session, or the session may be read from and written to using
    environment variables and HTTP headers, as appropriate.</p>

</summary>
<seealso><module>mod_session_cookie</module></seealso>
<seealso><module>mod_session_crypto</module></seealso>
<seealso><module>mod_session_dbd</module></seealso>

    <section id="whatisasession"><title>What is a session?</title>
      <p>At the core of the session interface is a table of key and value pairs
      that are made accessible across browser requests. These pairs can be set
      to any valid string, as needed by the application making use of the
      session.</p>

      <p>The "session" is a <strong>application/x-www-form-urlencoded</strong>
      string containing these key value pairs, as defined by the
      <a href="http://www.w3.org/TR/html4/">HTML specification</a>.</p>

      <p>The session can optionally be encrypted and base64 encoded before
      being written to the storage mechanism, as defined by the
      administrator.</p>

    </section>
    <section id="whocanuseasession"><title>Who can use a session?</title>
      <p>The session interface is primarily developed for the use by other
      server modules, such as <module>mod_auth_form</module>, however CGI
      based applications can optionally be granted access to the contents
      of the session via the HTTP_SESSION environment variable. Sessions
      have the option to be modified and/or updated by inserting an HTTP
      response header containing the new session parameters.</p>

    </section>
    <section id="serversession"><title>Keeping sessions on the server</title>
      <p>Apache can be configured to keep track of per user sessions stored
      on a particular server or group of servers. This functionality is
      similar to the sessions available in typical application servers.</p>

      <p>If configured, sessions are tracked through the use of a session ID that
      is stored inside a cookie, or extracted from the parameters embedded
      within the URL query string, as found in a typical GET request.</p>

      <p>As the contents of the session are stored exclusively on the server,
      there is an expectation of privacy of the contents of the session. This
      does have performance and resource implications should a large number
      of sessions be present, or where a large number of webservers have to
      share sessions with one another.</p>

      <p>The <module>mod_session_dbd</module> module allows the storage of user
      sessions within a SQL database via <module>mod_dbd</module>.</p>

    </section> <!-- /serversession -->

    <section id="browsersession"><title>Keeping sessions on the browser</title>
      <p>In high traffic environments where keeping track of a session on a
      server is too resource intensive or inconvenient, the option exists to store
      the contents of the session within a cookie on the client browser instead.</p>

      <p>This has the advantage that minimal resources are required on the
      server to keep track of sessions, and multiple servers within a server
      farm have no need to share session information.</p>

      <p>The contents of the session however are exposed to the client, with a
      corresponding risk of a loss of privacy. The
      <module>mod_session_crypto</module> module can be configured to encrypt the
      contents of the session before writing the session to the client.</p>

      <p>The <module>mod_session_cookie</module> allows the storage of user
      sessions on the browser within an HTTP cookie.</p>

    </section> <!-- /browsersession -->

    <section id="basicexamples"><title>Basic Examples</title>

      <p>Creating a session is as simple as turning the session on, and deciding
      where the session will be stored. In this example, the session will be
      stored on the browser, in a cookie called <code>session</code>.</p>

      <example><title>Browser based session</title>
      <highlight language="config">
Session On
SessionCookieName session path=/
        </highlight>
      </example>

      <p>The session is not useful unless it can be written to or read from. The
      following example shows how values can be injected into the session through
      the use of a predetermined HTTP response header called
      <code>X-Replace-Session</code>.</p>

      <example><title>Writing to a session</title>
      <highlight language="config">
Session On
SessionCookieName session path=/
SessionHeader X-Replace-Session
        </highlight>
      </example>

      <p>The header should contain name value pairs expressed in the same format
      as a query string in a URL, as in the example below. Setting a key to the
      empty string has the effect of removing that key from the session.</p>

      <example><title>CGI to write to a session</title>
      <highlight language="sh">
#!/bin/bash
echo "Content-Type: text/plain"
echo "X-Replace-Session: key1=foo&amp;key2=&amp;key3=bar"
echo
env
        </highlight>
      </example>

      <p>If configured, the session can be read back from the HTTP_SESSION
      environment variable. By default, the session is kept private, so this
      has to be explicitly turned on with the
      <directive module="mod_session">SessionEnv</directive> directive.</p>

      <example><title>Read from a session</title>
      <highlight language="config">
Session On
SessionEnv On
SessionCookieName session path=/
SessionHeader X-Replace-Session
        </highlight>
      </example>

      <p>Once read, the CGI variable <code>HTTP_SESSION</code> should contain
      the value <code>key1=foo&amp;key3=bar</code>.</p>

    </section>
    <section id="sessionprivacy"><title>Session Privacy</title>

      <p>Using the "show cookies" feature of your browser, you would have seen
      a clear text representation of the session. This could potentially be a
      problem should the end user need to be kept unaware of the contents of
      the session, or where a third party could gain unauthorised access to the
      data within the session.</p>

      <p>The contents of the session can be optionally encrypted before being
      placed on the browser using the <module>mod_session_crypto</module>
      module.</p>

      <example><title>Browser based encrypted session</title>
      <highlight language="config">
Session On
SessionCryptoPassphrase secret
SessionCookieName session path=/
        </highlight>
      </example>

      <p>The session will be automatically decrypted on load, and encrypted on
      save by Apache, the underlying application using the session need have
      no knowledge that encryption is taking place.</p>

      <p>Sessions stored on the server rather than on the browser can also be
      encrypted as needed, offering privacy where potentially sensitive
      information is being shared between webservers in a server farm using
      the <module>mod_session_dbd</module> module.</p>

    </section>
    <section id="cookieprivacy"><title>Cookie Privacy</title>

      <p>The HTTP cookie mechanism also offers privacy features, such as the
      ability to restrict cookie transport to SSL protected pages only, or
      to prevent browser based javascript from gaining access to the contents
      of the cookie.</p>

      <note type="warning"><title>Warning</title>
      <p>Some of the HTTP cookie privacy features are either non-standard, or
      are not implemented consistently across browsers. The session modules
      allow you to set cookie parameters, but it makes no guarantee that privacy
      will be respected by the browser. If security is a concern, use the
      <module>mod_session_crypto</module> to encrypt the contents of the session,
      or store the session on the server using the <module>mod_session_dbd</module>
      module.</p>
      </note>

      <p>Standard cookie parameters can be specified after the name of the cookie,
      as in the example below.</p>

      <example><title>Setting cookie parameters</title>
      <highlight language="config">
Session On
SessionCryptoPassphrase secret
SessionCookieName session path=/private;domain=example.com;httponly;secure;
        </highlight>
      </example>

      <p>In cases where the Apache server forms the frontend for backend origin servers,
      it is possible to have the session cookies removed from the incoming HTTP headers using
      the <directive module="mod_session_cookie">SessionCookieRemove</directive> directive.
      This keeps the contents of the session cookies from becoming accessible from the
      backend server.
      </p>

    </section>
    <section id="authentication"><title>Session Support for Authentication</title>

      <p>As is possible within many application servers, authentication modules can use
      a session for storing the username and password after login. The
      <module>mod_auth_form</module> saves the user's login name and password within
      the session.</p>

      <example><title>Form based authentication</title>
      <highlight language="config">
Session On
SessionCryptoPassphrase secret
SessionCookieName session path=/
AuthFormProvider file
AuthUserFile "conf/passwd"
AuthType form
AuthName "realm"
#...
        </highlight>
      </example>

      <p>See the <module>mod_auth_form</module> module for documentation and complete
      examples.</p>

    </section>
    <section id="integration"><title>Integrating Sessions with External Applications</title>

      <p>In order for sessions to be useful, it must be possible to share the contents
      of a session with external applications, and it must be possible for an
      external application to write a session of its own.</p>

      <p> A typical example might be an application that changes a user's password set by
      <module>mod_auth_form</module>. This application would need to read the current
      username and password from the session, make the required changes to the user's
      password, and then write the new password to the session in order to provide a
      seamless transition to the new password.</p>

      <p>A second example might involve an application that registers a new user for
      the first time. When registration is complete, the username and password is
      written to the session, providing a seamless transition to being logged in.</p>

      <dl>
      <dt>Apache modules</dt>
      <dd>Modules within the server that need access to the session can use the
      <strong>mod_session.h</strong> API in order to read from and write to the
      session. This mechanism is used by modules like <module>mod_auth_form</module>.
      </dd>

      <dt>CGI programs and scripting languages</dt>
      <dd>Applications that run within the webserver can optionally retrieve the
      value of the session from the <strong>HTTP_SESSION</strong> environment
      variable. The session should be encoded as a
      <strong>application/x-www-form-urlencoded</strong> string as described by the
      <a href="http://www.w3.org/TR/html4/">HTML specification</a>. The environment
      variable is controlled by the setting of the
      <directive module="mod_session">SessionEnv</directive> directive. The session
      can be written to by the script by returning a
      <strong>application/x-www-form-urlencoded</strong> response header with a name
      set by the <directive module="mod_session">SessionHeader</directive>
      directive. In both cases, any encryption or decryption, and the reading the
      session from or writing the session to the chosen storage mechanism is handled
      by the <module>mod_session</module> modules and corresponding configuration.
      </dd>

      <dt>Applications behind <module>mod_proxy</module></dt>
      <dd>If the <directive module="mod_session">SessionHeader</directive>
      directive is used to define an HTTP request header, the session, encoded as
      a <strong>application/x-www-form-urlencoded</strong> string, will be made
      available to the application. If the same header is provided in the response,
      the value of this response header will be used to replace the session. As
      above, any encryption or decryption, and the reading the session from or
      writing the session to the chosen storage mechanism is handled by the
      <module>mod_session</module> modules and corresponding configuration.</dd>

      <dt>Standalone applications</dt>
      <dd>Applications might choose to manipulate the session outside the control
      of the Apache HTTP server. In this case, it is the responsibility of the
      application to read the session from the chosen storage mechanism,
      decrypt the session, update the session, encrypt the session and write
      the session to the chosen storage mechanism, as appropriate.</dd>
      </dl>

    </section>

<directivesynopsis>
<name>Session</name>
<description>Enables a session for the current directory or location</description>
<syntax>Session On|Off</syntax>
<default>Session Off</default>
<contextlist><context>server config</context>
<context>virtual host</context>
<context>directory</context>
<context>.htaccess</context>
</contextlist>
<override>AuthConfig</override>

<usage>
    <p>The <directive>Session</directive> directive enables a session for the
    directory or location container. Further directives control where the
    session will be stored and how privacy is maintained.</p>
</usage>
</directivesynopsis>

<directivesynopsis>
<name>SessionMaxAge</name>
<description>Define a maximum age in seconds for a session</description>
<syntax>SessionMaxAge <var>maxage</var></syntax>
<default>SessionMaxAge 0</default>
<contextlist><context>server config</context>
<context>virtual host</context>
<context>directory</context>
<context>.htaccess</context>
</contextlist>
<override>AuthConfig</override>

<usage>
    <p>The <directive>SessionMaxAge</directive> directive defines a time limit
    for which a session will remain valid. When a session is saved, this time
    limit is reset and an existing session can be continued. If a session
    becomes older than this limit without a request to the server to refresh
    the session, the session will time out and be removed. Where a session is
    used to stored user login details, this has the effect of logging the user
    out automatically after the given time.</p>

    <p>Setting the maxage to zero disables session expiry.</p>
</usage>
</directivesynopsis>

<directivesynopsis>
<name>SessionEnv</name>
<description>Control whether the contents of the session are written to the
<var>HTTP_SESSION</var> environment variable</description>
<syntax>SessionEnv On|Off</syntax>
<default>SessionEnv Off</default>
<contextlist><context>server config</context>
<context>virtual host</context>
<context>directory</context>
<context>.htaccess</context>
</contextlist>
<override>AuthConfig</override>

<usage>
    <p>If set to <var>On</var>, the <directive>SessionEnv</directive> directive
    causes the contents of the session to be written to a CGI environment
    variable called <var>HTTP_SESSION</var>.</p>

    <p>The string is written in the URL query format, for example:</p>

    <example>
      <code>key1=foo&amp;key3=bar</code>
    </example>

</usage>
</directivesynopsis>

<directivesynopsis>
<name>SessionHeader</name>
<description>Import session updates from a given HTTP response header</description>
<syntax>SessionHeader <var>header</var></syntax>
<default>none</default>
<contextlist><context>server config</context>
<context>virtual host</context>
<context>directory</context>
<context>.htaccess</context>
</contextlist>
<override>AuthConfig</override>

<usage>
    <p>The <directive>SessionHeader</directive> directive defines the name of an
    HTTP response header which, if present, will be parsed and written to the
    current session.</p>

    <p>The header value is expected to be in the URL query format, for example:</p>

    <example>
      <code>key1=foo&amp;key2=&amp;key3=bar</code>
    </example>

    <p>Where a key is set to the empty string, that key will be removed from the
    session.</p>

</usage>
</directivesynopsis>

<directivesynopsis>
<name>SessionInclude</name>
<description>Define URL prefixes for which a session is valid</description>
<syntax>SessionInclude <var>path</var></syntax>
<default>all URLs</default>
<contextlist><context>server config</context>
<context>virtual host</context>
<context>directory</context>
<context>.htaccess</context>
</contextlist>
<override>AuthConfig</override>

<usage>
    <p>The <directive>SessionInclude</directive> directive allows sessions to
    be made valid for specific URL prefixes only. This can be used to make a
    website more efficient, by targeting a more precise URL space for which
    a session should be maintained. By default, all URLs within the directory
    or location are included in the session.</p>

    <note type="warning"><title>Warning</title>
    <p>This directive has a similar purpose to the <var>path</var> attribute
    in HTTP cookies, but should not be confused with this attribute. This
    directive does not set the <var>path</var> attribute, which must be
    configured separately.</p></note>
</usage>
</directivesynopsis>

<directivesynopsis>
<name>SessionExclude</name>
<description>Define URL prefixes for which a session is ignored</description>
<syntax>SessionExclude <var>path</var></syntax>
<default>none</default>
<contextlist><context>server config</context>
<context>virtual host</context>
<context>directory</context>
<context>.htaccess</context>
</contextlist>
<override>AuthConfig</override>

<usage>
    <p>The <directive>SessionExclude</directive> directive allows sessions to
    be disabled relative to URL prefixes only. This can be used to make a
    website more efficient, by targeting a more precise URL space for which
    a session should be maintained. By default, all URLs within the directory
    or location are included in the session. The
    <directive module="mod_session">SessionExclude</directive> directive takes
    precedence over the
    <directive module="mod_session">SessionInclude</directive> directive.</p>

    <note type="warning"><title>Warning</title>
    <p>This directive has a similar purpose to the <var>path</var> attribute
    in HTTP cookies, but should not be confused with this attribute. This
    directive does not set the <var>path</var> attribute, which must be
    configured separately.</p></note>
</usage>
</directivesynopsis>

<directivesynopsis>
<name>SessionExpiryUpdateInterval</name>
<description>Define the number of seconds a session's expiry may change without
the session being updated</description>
<syntax>SessionExpiryUpdateInterval <var>interval</var></syntax>
<default>SessionExpiryUpdateInterval 0 (always update)</default>
<contextlist><context>server config</context>
<context>virtual host</context>
<context>directory</context>
<context>.htaccess</context>
</contextlist>
<override>AuthConfig</override>

<usage>
    <p>The <directive>SessionExpiryUpdateInterval</directive> directive allows
    sessions to avoid the cost associated with writing the session each request
    when only the expiry time has changed. This can be used to make a website
    more efficient or reduce load on a database when using
    <module>mod_session_dbd</module>. The session is always written if the data
    stored in the session has changed or the expiry has changed by more than the
    configured interval.</p>

    <p>Setting the interval to zero disables this directive, and the session
    expiry is refreshed for each request.</p>

    <p>This directive only has an effect when combined with
    <directive module="mod_session">SessionMaxAge</directive> to enable session
    expiry. Sessions without an expiry are only written when the data stored in
    the session has changed.</p>

    <note type="warning"><title>Warning</title>
    <p>Because the session expiry may not be refreshed with each request, it's
    possible for sessions to expire up to <var>interval</var> seconds early.
    Using a small interval usually provides sufficient savings while having a
    minimal effect on expiry resolution.</p></note>
</usage>
</directivesynopsis>

</modulesynopsis>